The present invention relates to a slit-exposure type illumination apparatus for use in electrophotographic copying machines or the like. Conventionally, in a slit-exposure type illumination apparatus employing a cylindrical reflector having a quadratic surface and a linear light surface such as a halogen lamp, correction for reduced illumination in the peripheral portions of the image-formation optical system is achieved by varying or altering the location of the light source or the light-emitting portions thereof, or by utilizing side reflectors on opposite sides of each light-emitting portion. However, these methods are applicable only when the lengths of the optical paths from the light source to all the original scanning portions are equal. These conventional methods of correction are not applicable when the lengths of the optical paths vary depending upon the scanning position as in the case of a "search light" type original scanning apparatus. In such apparatus, the change in length of the optical path will bring about a change in distribution of illuminance, and thus a change in image density in the various portions of the original document being copied.